Toroidal midplane neutral beam armor and plasma limiter

ABSTRACT

For use in a tokamak fusion reactor having a midplane magnetic coil on the inner wall of an evacuated toriodal chamber within which a neutral beam heated, fusing plasma is magnetically confined, a neutral beam armor shield and plasma limiter is provided on the inner wall of the toroidal chamber to shield the midplane coil from neutral beam shine-thru and plasma deposition. The armor shield/plasma limiter forms a semicircular enclosure around the midplane coil with the outer surface of the armor shield/plasma limiter shaped to match, as closely as practical, the inner limiting magnetic flux surface of the toroidally confined, indented, bean-shaped plasma. The armor shield/plasma limiter includes a plurality of semicircular graphite plates each having a pair of coupled upper and lower sections with each plate positioned in intimate contact with an adjacent plate on each side thereof so as to form a closed, planar structure around the entire outer periphery of the circular midplane coil. The upper and lower plate sections are adapted for coupling to heat sensing thermocouples and to a circulating water conduit system for cooling the armor shield/plasma limiter.The inner center portion of each graphite plate is adapted to receive and enclose a section of a circular diagnostic magnetic flux loop so as to minimize the power from the plasma confinement chamber incident upon the flux loop.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention under ContractNo. DE-AC02-76-CH03073 between the U.S. Department of Energy andPrinceton University.

BACKGROUND OF THE INVENTION

This invention relates generally to nuclear fusion reactors of thetokamak type and is particularly directed to a neutral beam shield andplasma limiter for use in a tokamak fusion reactor.

Among the various approaches currently under evaluation as a potentiallong term source of energy produced by nuclear fusion is the magneticconfinement of an energetic plasma in the form of a toroid or"doughnut". The apparatus used in the confinement, excitation andextraction of energy from the plasma is known as a tokamak fusionreactor. A tokamak fusion reactor includes a circular arrangement ofpowerful magnets for generating a toroidal magnetic field wherein anenergetic plasma comprised primarily of protons, deuterons, tritons andelectrons is confined. The toroidally confined plasma may be energizedby various means including the injection of energetic neutral particles,typically deuterium, not influenced by the magnetic field which are ableto penetrate into and heat the plasma. The thermal energy thus producedwithin the plasma causes the nuclei therein to fuse with the release ofsubstantial energy.

The confining magnetic field of a tokamak reactor is generally toroidalwith the magnetic field lines made to spiral in the toroidal directionby a poloidal field produced by current flowing within the plasma. Thisplasma current can also be used to produce ohmic heating of the plasmawhich is generally supplemented by means of the aforementioned neutralbeam injection of energetic particles to attain those temperaturesnecessary for the fusion of nuclei. Supplementing of the ohmic heatingof the plasma with energetic neutral particles is necessary primarilydue to a decrease in the resistance of the plasma with increasingtemperatures.

Although the plasma of a tokamak reactor is confined by a magneticfield, the vacuum chamber within which the plasma is generated andconfined typically includes various structures for defining the size andshape of the confined plasma. These structures are generally referred toas plasma limiters, or limiter blades, and they may either be fixed ormovable within the vacuum chamber.

Whether stationary or movable, a plasma limiter must be capable ofwithstanding not only tremedous thermal loads, but also extremely largeelectromagnetic forces and mechanical vibrations arising from the pulsednature in which the plasma is energized. In addition, substantialmechanical loading of the plasma limiter may result from plasmadisruptions as eddy currents induced in the plasma limiter react withthe rapidly changing magnetic field. These plasma limiters not onlyfunction to form the plasma in a desired shape, but also serve to shieldvarious components within the tokamak reactor from the hostileenvironment within the plasma chamber. For example, variouselectromagnetic and thermal sensors are provided around the periphery ofthe plasma chamber in order to monitor the various parameters of theenvironment therein representing the behavior and characteristics of theheated plasma. In performing this function, the limiter must provideprotection for a given sensor, while orienting and positioning thesensor for maximum sensitivity. The plasma limiter must also provideshielding for the various coils producing the confining magnetic fieldwhile allowing for their proper positioning relative to the toroidalvacuum chamber.

The present invention provides all of the aforementioned features in atoroidal midplane neutral beam armor and plasma limiter which ispositioned on the inner wall of a tokamak fusion reactor for shielding aa midplane magnetic coil while allowing a magnetic flux loop sensor tobe positioned in close proximity to the heated plasma. The armorshield/plasma limiter of the present invention is uniquely adapted towithstand the extreme thermal loads, electromagnetic forces, andmechanical stress present in a pulsed tokamak fusion reactor whileproviding the required plasma control and an ultra clean environmentwithin the plasma chamber.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provideimproved plasma confinement, reaction diagnostics and thermal isolationin a tokamak fusion reactor.

It is another object of the present invention to provide a combinedneutral beam shield and plasma limiter for use in a tokamak fusionreactor wherein is magnetically confined a bean-shaped fusing plasma.

Yet another object of the present invention is to provide within thereaction chamber of a tokamak fusion reactor an inner wall thermalarmor, a neutral beam shield and a large area inner toroidal plasmalimiter.

A further object of the present invention is to provide a toroidalmidplane plasma limiter and neutral beam armor structure for a neutralbeam heated, indented, bean-shaped plasma magnetically confined in atokamak-type fusion reactor.

A still further object of the present invention is to provide improvedthermal, electromagnetic and mechanical protection and isolation in atokamak fusion reactor.

Another object of the present invention is to provide a neutral beamarmor/plasma limiting structure for use in a tokamak fusion reactorwhich affords improved plasma isolation and confinement, increasedprotection for reactor components and structure adjacent to thereactors's plasma confinement chamber, and enhanced structuralintegrity, reliability and safety.

Accordingly, the present invention contemplates an armor shield/plasmalimiter positioned upon the inner wall of a toroidal vacuum chamberwithin which is magnetically confined an energetic plasma in a tokamaknuclear fusion reactor. The armor shield/plasma limiter is thus of ageneral semi-toroidal shape and is comprised of a plurality of adjacentgraphite plates positioned immediately adjacent to each other so as toform a continuous ring upon and around the toroidal chamber's inner walland the reactor's midplane coil. Each plate has a generallysemi-circular outer circumference and a recessed inner portion and iscomprised of upper and lower half sections positioned immediatelyadjacent to one another along the midplane of the plate. With the upperand lower half sections thus joined, a channel or duct is providedwithin the midplane of the plate in which a magnetic flux loop ispositioned. The magnetic flux loop is thus positioned immediatelyadjacent to the fusing toroidal plasma and serves as a diagnostic sensorwith the armor shield/plasma limiter minimizing the amount of power fromthe energetic plasma as well as from the neutral particle beams heatingthe plasma incident upon the flux loop. The outer curvature of each ofthe plates is selected to provide a toroidal surface which matches, asclosely as practical, a selected inner limiting magnetic flux surface ofthe fusing plasma based upon plasma shaping considerations. Thethickness of the plate is such as to minimize the distance between themidplane magnetic flux coil and the heated plasma, while maximizing thematerial thickness therebetween so as to provide sufficient mechanicalstrength and thermal isolation for the magnetic flux coil.

The upper and lower end portions of each plate are adapted for couplingto thermocouples for temperature measurements while the recessed innerportion of the graphite plate is adapted for coupling to water cooledplates by means of T-bars. The front surface of each plate is wider thanits rear surface similar to the shape of a pie wedge to minimize thenumber of required plates while providing an approximately circularouter surface to the toroidal plasma. The armor shield/plasma limiter ofthe present invention is designed to withstand the thermal loads,electromagnetic forces, mechanical vibrations and duty cycle of thefusing plasma while providing plasma control and a containment-freeenvironment within the plasma chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended claims set forth those novel features which characterizethe invention. However, the invention itself, as well as further objectsand advantages thereof, will best be understood by reference to thefollowing detailed description of a preferred embodiment taken inconjunction with the accompanying drawings, where like referencecharacters identify like elements throughout the various figures, inwhich:

FIG. 1 is a horizontal sectional view in simplified schematic diagramform of a toroidal-shaped tokamak fusion reactor taken along themidplane thereof illustrating the position of the armor shield/plasmalimiter of the present invention therein;

FIG. 2 is a perspective view of the toroidal plasma chamber within atokamak fusion reactor showing the armor shield/plasma limiter of thepresent invention positioned therein;

FIG. 3 is an outer side view of a plate for use in the armorshield/plasma limiter of the present invention; and

FIG. 4 is an inner side view of the plate shown in FIG. 3 as used in thearmor shield/plasma limiter of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Referring to FIG. 1, there is shown a horizontal sectional view insimplified diagrammatic form taken along the midplane of a tokamakreactor 10 with which the armor shield/plasma limiter 28 of the presentinvention is intended for use. The tokamak reactor 10 includes atoroidal plasma chamber 12 defined by circular outer and inner walls 14,16, a lower wall 34, and an upper wall which is not shown in FIG. 1.Confined within the plasma chamber 12 by means of a magnetic field is aplasma 26 comprised primarily of protons, deuterons, tritons andelectrons. A toroidal magnetic field is generated by a circulararrangement of magnetic coils which are not shown in FIG. 1 forsimplicity. The plasma 26 is energized by ohmic heating and by injectingneutral particles therein from a plurality of neutral beam sources 18,20, 22 and 24. These neutral particles may be comprised of lithium atomsor helium atoms or any other similar, relatively light, neutralizedatomic particles, with deuterium particles used in a preferredembodiment. The energetic plasma is typically comprised of approximately90% deutrons and tritons and 10% helium ions. The high energies of theparticles within the plasma cause the fusing of atoms, such as deuteriumand tritium, and the resulting production of energy. The neutralparticle beams may be directed into the energetic plasma over a widerange of orientations relative to the direction of current flow as shownby the direction of the arrows therein, with the orientation of theneutral particle beams as shown in FIG. 1 shown generally at 90°relative to the direction of current flow for illustrative purposes.

Referring to FIG. 2, there is shown a perspective view of the tokamakreactor from within its toroidal plasma chamber 12. As shown in FIG. 2,the plasma chamber 12 is defined by the aforementioned outer and innerwalls 14, 16 as well as upper and lower walls 32, 34 to provide agenerally doughnut-shaped space within the reactor. It is within thisspace that the fusing plasma is confined by means of a plurality ofmagnetic coils oriented about the space defined by the aforementionedwalls.

Various magnetic coils are positioned within and about the toroidalplasma chamber 12. One such magnetic coil is termed the midplane coil 44which is positioned in the midplane of the plasma chamber 12 and ismounted to the inner wall 16 thereof. The midplane coil 44 is used toprovide the toroidal plasma with an indented, bean-shaped cross section.Positioned immediately outward from and around the length of themidplane coil 44 is the armor shield/plasma limiter 28 of the presentinvention. The armor shield/plasma limiter 28 is provided with agenerally circular outer surface chosen to match, as closely aspractical, a selected inner limiting magnetic flux surface of the fusingplasma as determined from plasma shaping consideration. The detailedcurvature of the outer surface of the armor shield/plasma limiter 28 isalso a function of the design parameters of the tokamak reactor.

In a preferred embodiment, the armor shield/plasma limiter 28 isprovided with a semi-circular outer surface of appropriate radius toclosely approximate true tangency with the selected inner limitingmagnetic flux surface thus allowing the use of conventional machiningtechniques during fabrication. Generally, such plasma limiters must beconditioned with high power plasma depositions for relatively long andinconvenient durations in order to render the plasma limiter suitablefor limiting high temperature, low impurity plasmas. Therefore, in apreferred embodiment, the armor shield/plasma limiter surface whichcontacts the plasma was machined to a No. 32 finish and was mechanicallyworked to a relatively high polish. This procedure reduced the plasmalimiter conditioning time by minimizing the number of micro-surfacestructures which could ablate and result in the introduction ofimpurities within the plasma. Thus, in a preferred embodiment, the armorshield/plasma limiter 28 is comprised of an uncoated ATJ graphite.

Also positioned on the inner wall 16 of the toroidal plasma chamber 12are upper and lower thermocouples 40, 42 which are adapted for couplingto respective upper and lower inner ends of the armor shield/plasmalimiter 28. The upper and lower thermocouples 40, 42 are responsive tothe temperature of the armor shield/plasma limiter 28. The temperatureof the armor shield/plasma limiter 28, is, in turn, responsive to andprovides an indication of the intensity of the neutral beam injectioninto the toroidal plasma chamber 12. Therefore, the upper and lowerthermocouples 40, 42, while directly responsive to the temperaturewithin the armor shield/plasma limiter 28, provide an accurate indirectmeasure of the neutral beam energy injected into the toroidal plasmachamber 12 for heating the magnetically confined plasma therein.

Also positioned upon and about the inner wall 16 and within the armorshield/plasma limiter 28 around the length thereof are a plurality ofwater cooled support plates 38. The circulating water within the watercooled support plates 38 is used to remove excess thermal energy fromthe armor shield/plasma limiter 28. By regulating the flow of waterwithin the water cooled support plates 38, the temperature of the armorshield/plasma limiter 28 may be maintained at a safe level even duringplasma instabilities and with the introduction of high energies into thetoroidal plasma chamber 12 by the neutral beam sources. The upper andlower thermocouples 40, 42 may be connected to the water cooled supportplates 38 by a conventional flow control system (not shown) to permitthe rate of flow of coolant within the support plates to be controlledin accordance with the temperature of the armor shield/plasma limiter28. Also positioned about the inner wall 16 immediately outside of themidplane coil 44 is a toroidally symmetric, diagnostic magnetic fluxloop 36 which is responsive to and provides a signal indicative of thestrength of the magnetic field immediately adjacent to an inner portionof the toroidally shaped plasma. The midplane magnetic flux loop 36 isenclosed within a thin stainless steel tube and is positioned within andalong the length of the armor shield/plasma limiter 28 on the midplanethereof as described in detail below.

As shown in FIG. 2, the armor shield/plasma limiter 28 is comprised of aplurality of plates 30 positioned immediately adjacent to one another ina planar, circular arrangement around the inner wall 16. Additionaldetails of these plates 30 can be seen from FIGS. 3 and 4 whichrespectively show outer and inner side views of a plate 30 as used inthe armor shield/plasma limiter 28 of the present invention. Each plate30 has a generally circular outer surface and a recessed inner portion.The width of the plate 30 is such as to minimize the number of requiredplates positioned around the vacuum chamber's inner wall 16 whileproviding an approximately circular outer surface around thecircumference of the armor shield/plasma limiter 28 to the toroidalplasma. Thus, the front, outer surface of each plate is wider than therear, inner surface thereof in a manner analagous to a pie wedge.

Referring specifically to FIGS. 3 and 4, each plate 30 is comprised ofan upper section 30a and a lower section 30b. The upper and lower platesections 30a, 30b are positioned in abutting contact along the midplaneof the plate 30. The region of maximum power deposition upon the plate30 is along the midplane where the upper and lower sections 30a, 30bmate. The material thickness in the midplane region of the plate 30 ischosen so as to minimize the distance between the midplane coil, whichis positioned aft or in the recessed portion of the plate 30, and theplasma, while maximizing the material thickness between the midplanecoil and the plasma so as to provide sufficient mechanical strength andthermal inertia or isolation therebetween. In the region of minimumthickness of the upper and lower plate sections 30a, 30b, which islocated midway between the rear and midplane portions of the platesections, sufficient clearance is provided between the midplane coil andthe armor shield/plasma limiter 28 so as to provide electricalinsulation therebetween, while maximizing the thickness in these regionsso as to increase heat conduction to the heat extraction regions of eachplate section located adjacent to the T-slots, which are described indetail below, therein. Those portions located midway between the rearand midplane portions of each of the plate sections are also shaped soas to minimize sharp corners on the inner surface thereof in order toreduce, as much as possible, regions of high stress which could initiateand propagate a material failure under the extreme conditions within thetokamak fusion reactor. This operating characteristic is particularlyimportant in the armor shield/plasma limiter 28 of the present inventionsince inner plasma limiter systems are subject to large magnetic forcesand high power densities when disruptions occur in the plasma current.

Forward and aft T-slots 58, 60 are provided in the rear surface of thelower section 30b adjacent to the aft end thereof. Similarly, forwardand aft T-slots 66, 68 are provided in the rear surface of the uppersection 30a of the plate 30 adjacent to the aft end thereof. Each of theT-slots 58, 60, 66 and 68 is characterized as having a respectiveenlarged inner portion 62, 64, 70 and 72. With the various plates 30within the armor shield/plasma limiter 28 positioned in abutting lateralcontact with adjacent plates in an aligned array, the respective T-slotsof adjacent plates from continuous T-shaped channels around the entirecircular length of the armor shield/plasma limiter. The continuousarrangement of T-slots within each of the upper and lower sections 30a,30b of the plates positioned around the armor shield/plasma limiter areadapted to receive a respective T-bar. An upper T-bar 55 is shown indotted line form inserted within the aft T-slot 68 within the uppersection 30a of the plate 30. Similarly, a lower T-bar 57 is shown indotted line form positioned within the aft T-slot 60 within the lowersection 30b of the plate 30. Similar T-bars may be positioned within theforward T-slots 58, 66 of the lower and upper sections 30b, 30a,although these T-bars are not shown in the figure for simplicity.

The T-slots in both the upper and lower sections 30a, 30b allow theplate 30 to be mounted to the water cooled support plates 38 shown inFIG. 2 by means of a plurality of T-bars. Stress calculations indicatethat this manner of supporting the individual plates 30 of the armorshield/plasma limiter 28 minimizes thermal stress while providingsufficiently high thermal conductivity between the graphite plates andthe aforementioned cooling plates. Each of the T-bars inserted within arespective T-slot within the plate 30 may be securely coupled to or maybe a part of a respective water cooled support plate 38 which alsoserves to support and maintain in position the upper and lower sections30a, 30b of the plate.

The upper end 48 of the upper section 30a and the lower end 50 of thelower section 30b of the plate 30 are each provided with a respectiveaperture 74, 76 therein which is adapted to receive and engage arespective thermocouple 78, 76. Each of the thermocouples 78, 76 isresponsive to the temperature within that portion of the plate 30 towhich it is coupled. The upper and lower thermocouples 78, 80 are notintended to provide an indication of temperature changes after eachplasma pulse within the tokamak reactor, but rather are designed tomeasure the effect of temperature ratcheting after a series of plasmapulses within the reactor. Temperature ratcheting is caused by thethermal build up following each successive plasma pulse within thereactor which is unable to completely dissipate this cumulative heatingeffect and which thus undergoes a step-like increase in temperature inresponsive to a series of plasma pulses.

Referring to the midplane portion of the plate 30 as shown in FIGS. 3and 4, a small channel or duct 54 is provided between the upper andlower plate sections 30a, 30b. This gap 54 is sized so as to providerelief for thermal expansions of those portions of the upper and lowerplate sections 30a, 30b in the midplane region during maximum powerdeposition while minimizing the amount of power which enters the gap. Athickened portion at the lower end of the upper plate section 30a isthus positioned between the channel 54 and the outer, circular portionof the plate 30 which is in contact with and shapes the plasma in a beanconfiguration.

The channel 54 between the upper and lower plate sections 30a, 30bprovides clearance for the positioning of a thin stainless steel tube(not shown) within which is positioned the toroidally symmetric,diagnostic magnetic flux loop 36. The diagnostic magnetic flux loop 36is responsive to the charge flow within the plasma and is of criticalimportance for monitoring and controlling the indented, bean-shapedplasma located immediately adjacent to the outer surface of the plates30. The overlapping of the upper plate section 30a of the channel 54within which is positioned the diagnostic magnetic flux loop 36minimizes the amount of power from the plasma chamber incident upon theflux loop. It is essential to minimize the power incident upon thediagnostic magnetic flux loop 36 because of the relatively poor thermalconductivity of the stainless steel tube enclosing the flux loop whichcould reach temperatures sufficiently high to damage the flux loop. Theoverlapping configurations of the upper and lower plate sections 30a,30b allow the diagnostic magnetic flux loop 36 to be positioned as closeto the toroidal plasma as possible while providing sufficient graphitethickness of the plate 30 between the diagnostic magnetic flux loop andthe toroidal plasma for the required mechanical strength and thermalshielding. The notches 56 on the inner portion of the plate 30 adjacentto the midplane thereof provide sufficient clearance for a small bondingprotrusion on the vacuum enclosure of the midplane coil 44 to which theupper and lower plate sections 30a, 30b may be securely mounted.

There has thus been shown a combined armor shield/plasma limiter for useon the inner wall of an evacuated toroidal chamber of a tokamak fusionreactor which serves to shape a fusing plasma therein in an indented,bean-shaped configuration while providing the necessary shielding forpreventing neutral beam shine-through and plasma deposition upon thereactor's midplane magnetic coil. The armor shield/plasma limiter isadapted to provide an indication of the thermal energy stored in theenergized plasma and to provide support and shielding for a diagnosticmagnetic flux loop responsive to plasma activity within the evacuatedtoroidal chamber.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications may be made without departing from theinvention in its broader aspects. For example, while the armorshield/plasma limiter has been disclosed as comprising a plurality ofconnected, abutting individual plates completely surrounding themidplane magnetic coil on the inner wall of the toroidal vacuum chamber,the present invention also envisions placing only a limited number ofgroups of abutting armor shield/plasma limiter plates about portions ofthe plasma chamber's inner wall so as to shield the midplane coil fromone or more neutral beam sources injecting energetic neutrals into theplasma chamber. Therefore, the aim in the appended claims is to coverall such changes and modifications as fall within the true spirit andscope of the invention. The matter set forth in the foregoingdescription and accompanying drawings is offered by way of illustrationonly and not as a limitation. The actual scope of the invention isintended to be defined in the following claims when viewed in theirproper perspective based on the prior art.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. In a tokamak fusionreactor having a plurality of coils including a midplane coil forgenerating a magnetic field within an evacuated toroidal chamber havingan inner wall and wherein is confined a toroidal plasma into which isdirected a beam of high energy neutral particles for energizing saidplasma, said tokamak reactor further including a midplane diagnosticmagnetic flux loop for analyzing said plasma, an armor shield and plasmalimiter for shielding said midplane coil from said neutral particle beamand for forming said plasma into an indented, bean-shaped configurationcomprising:a plurality of plates mounted to the inner wall of the plasmachamber about at least a portion of the length thereof and positionedimmediately adjacent to and around the outer periphery of the midplanecoil along at least a portion of the length thereof, wherein each ofsaid plates is positioned in lateral abutting contact with at least oneother plate so as to form a generally planar, closed structure around atleast a portion of the length of the midplane coil, wherein each of saidplates includes:an upper section having upper and lower end portions andincluding a generally one quarter circular outer portion directed towardthe plasma chamber and an inner recessed portion directed toward theinner wall; and a lower section having upper and lower end portions andincluding a generally one quarter circular outer portion directed towardthe plasma chamber and an inner recessed portion directed toward theinner wall, wherein the lower end portion of said upper section ispositioned in abutting contact with the upper end portion of said lowersection along the midplane of the tokamak rector so as to define achannel between said upper and lower sections within which is positionedthe diagnostic magnetic flux loop and to form a generally semi-circularplate having an outer circular portion in contact with the plasma and aninner recessed portion within which is positioned the midplane coil forshielding from the neutral particle beam.